In general terms, in order to explore hydrocarbon deposits, it is highly desirable to obtain accurate knowledge of the characteristics of the geological formation at various depths of the borehole. Many of these characteristics are very fine in structure, e.g. stratifications, non-uniform elements, pore characteristics, breaks, etc. By way of example, the orientations, the density, and the lengths of breaks play a major role in the dynamic characteristics of a reservoir rock.
For many years, it has been possible to determine such fine characteristics only by analyzing drill cores taken when drilling the borehole. However the taking of such cores is a technique that is extremely expensive, and use thereof remains relatively exceptional.
Patent EP-0 110 750, or corresponding U.S. Pat. No. 4,567,759, describes a technique of producing an image of the wall of a borehole which consists in generating characteristic signals at regular time intervals representative of a measurement with high spatial resolution of some characteristic of the wall, measuring the depth of the hole to which the characteristic signals relate with accuracy of the same order as the spatial resolution of the characteristic signals, and converting the signals representing the characteristic as a linear function of borehole depth with a color scale being associated with the values of the converted signals in order to form a visual image.
That imaging technique is implemented more particularly with a tool for investigating the conductivity of the formation, as described for example in patent EP-0 071 540, or its corresponding U.S. Pat. No. 4,468,623, that is capable of detecting characteristics with millimeter resolution. That type of tool has a series of control electrodes, also known as “buttons”, placed on a conductive pad pressed against the wall of the borehole. A constant current source applies voltage to each button and the conductive surface of the pad so that measurement currents are injected into the formation perpendicularly to the wall. A return is provided for the current by means of an electrode situated close to the surface, or possibly on another part of the tool. The pad is moved along the borehole and the discrete currents associated with each button are proportional to the conductivity of the material facing the buttons.
In application of the teaching of patent U.S. Pat. No. 4,567,759, the signals are modified by eliminating effects such as variations in the speed of the tool and disturbances due to variations in the environment of the tool as amplified and displayed in a manner which comes close to providing a visual image of the inside of the hole.
That imaging technique has been highly successful over the last few years when used in boreholes drilled with conductive drilling mud such as water-base mud or mud of the oil-in-water emulsion type. However, with muds having a continuous non-conductive phase, such as oil-base muds or water-in-oil emulsion type muds, the images obtained are of very poor quality. These poor results are generally attributed to interference due to the presence of a layer of non-conductive mud, or of a layer of mud and a mud cake, interposed between the buttons and the formation under test. Since the thickness of the layer of mud varies in particular as a function of the roughness of the wall, the variations in the resulting currents can completely mask any current variations due to the formation being measured.
A novel technique for imaging a borehole drilled with a non-conductive drilling mud is described in international patent application PCT/US99/14420. That tool for investigating formation conductivity differs from the tool known from patent U.S. Pat. No. 4,468,623 in that it uses a non-conductive pad and buttons that form voltage electrodes instead of current electrodes. The current injection electrodes are situated off the pad, or in a preferred variant, directly at the ends thereof. In any event, the two-injectors are placed in such a manner that current passes through the formation substantially parallel to the pad and thus preferably flows substantially orthogonally to the boundaries of the strata. Under such conditions, the potential difference between two buttons is proportional to the resistivity of the material facing the buttons.
The above-specified international patent application recommends using DC, or AC at very low frequency, such that the resistivity of the pad is much greater than the resistivity of the drilling mud. However, in practice, working with DC gives rise to problems of noise due in particular to the formation of spontaneous potentials in the formation. In addition, the quantity of current injected is limited by the resistance of the mud; the potential differences measured between two pairs of buttons are thus very small and therefore difficult to measure.
It would therefore be desirable for it to be possible to work with AC at a relatively high frequency, e.g. of the order of a few thousand hertz. Unfortunately, at such frequencies, the pad behaves like a dielectric and the effective conductivity of the mud also increases. This gives rise to an electrical impedance through the pad that is of the same order as the impedance through the layer of mud. Under such conditions, the potential differences between pairs of buttons are more representative of the potential difference applied between the current electrodes than they are of the resistivity of the formation facing them, and as a result the tool becomes unusable.